1) Goal: To determine the precise nature of damage in macromolecules caused directly by ionizing radiation and to utilize this knowledge in the application and extension of radiation target analysis. 2) Importance: Radiation target analysis is the only method which can determine in vivo the mass of molecular structures required for a specific biochemical process. 3) Recent progress: The size of several different proteins which are active in biological processes was determined by the radiation technique and reported. These include enzymes (phenylalanine hydroxylase, glutathione S-transferase, human hepatic lipase, and the transcription termination protein Rho) and receptors (sigma1). The first radiation target analysis of a ribozyme was reported.